There is an entire market today which requires parts with weights less than 1 g or with greater weights with microstructural details. The electrical engineering, medical, telecommunications, automotive, biotechnological and aerospace markets etc. are developing new applications involving the manufacture of parts with very small dimensions due to the generalized trend towards miniaturizing products.
Some devices are known in the state of the art which use ultrasound to make it easier to expel parts from the mold, as well as to maintain a process temperature in the mold, even to homogenize the mass previously melted by the conventional system of resistors in the nozzle of the plasticizer. The conventional system consists of an assembly of heating resistors, placed in the plasticizing cylinder which is responsible for heating and melting the thermoplastic.
In most cases, microinjection machines on the market are simply an escalation of large conventional injection machines.
Patent EP-A-0930144 with its equivalent U.S. Pat. No. 6,203,747 describes an injection machine aided by ultrasound and the method for using it. Said machine includes an injection cylinder, a typical material conveyance unit, a screw, a piston, and a vibration element connected to said cylinder to produce relative movement between the cylinder and said material conveyance unit. In this invention, the material to be melted is introduced in the cylinder, where it is melted, and is subsequently homogenized by the action of the vibrations.
The mentioned patent does not describe the use of ultrasound to cause the thermoplastic material to completely melt for the injection almost instantaneously, but rather the plastic is previously melted by means of other methods, such as by means of a thermal element, for example, in the case of the analyzed patent, and the ultrasound is applied to mitigate or correct the lack of homogeneity caused by the long time elapsing in said melting process.
Microinjection machines known on the market generally heat the thermoplastic material supplied in the form of granules or microgranules inside a plasticizing cylinder in which, upon the rotation of a screw, the material is cyclically metered discontinuously and conveyed and melted in order to inject it directly into the mold. Another microinjection system assembles an independent extrusion screw which melts the plastic and passes it to a second chamber to be injected into the mold by means of a piston.
In both cases, and especially when dealing with micro parts, due to the little injection volume to be made in each piston stroke, there is a serious problem for the polymer if the dwell time of the molten granules inside the plasticizer exceeds their residence capacity in this medium, which is the cause of the molecular degradation of the plastic and of the subsequent reduction of quality and loss in the mechanical characterization of the injected parts, which is aggravated when a larger injectable volume is available in the machines.
W. Michaeli, A. Spennemann, R. Gatner (2002), New plastification concepts for micro injection molding, Microsystem Technologies 8, 55-57, Springer-Verlag 2002, describes a proposal for plasticizing micro parts by ultrasound and it describes a testing unit. This reference indicates that different parameters of the machine were measured by means of this unit, parameters such as performance of the ultrasound generator, extent and course of the sonotrode, activation power, etc. However, this document does not indicate particular constructive details or details relating to the manner of assembling the sonotrode, and it even raises questions as to assessing how ultrasonic plasticization can be integrated in current injection molding machines.